With the recent widespread use of cordless equipment such as a personal computer or portable telephone, secondary batteries used as a power source of cordless equipment are increasingly required to have a smaller size and higher capacity. At present, a lithium ion secondary battery that can achieve a small size, light weight, and high energy density is being put to practical use and growing in demand as a portable power source. However, depending on the type of cordless equipment to be used, the lithium ion secondary battery is not yet reliable enough to ensure a continuous available time.
Under these circumstances, a polymer electrolyte fuel cell has been studied as an example of a battery that may meet the above requirements. The polymer electrolyte fuel cell uses a polymer electrolyte membrane as an electrolyte, oxygen in the air as a positive active material, and a fuel (hydrogen, methanol, etc.) as a negative active material, and has attracted considerable attention because it is a battery that can be expected to have a higher energy density than a lithium ion secondary battery. Fuel cells can be used continuously as long as a fuel and oxygen are supplied. Although there are several candidates for fuels used for the fuel cells, the individual fuels have various problems, and a final decision has not been made yet.
For example, when a fuel cell uses hydrogen as a fuel, a method for supplying hydrogen stored in a high-pressure tank or hydrogen-storing alloy tank is employed to some extent. However, a fuel cell using such a tank is not suitable for a portable power source, since both the volume and the weight of the fuel cell are increased, and the energy density is reduced.
When a fuel cell uses a hydrocarbon fuel, another method for extracting hydrogen by reforming the hydrocarbon fuel may be employed. However, a fuel cell using hydrocarbon fuel requires a reformer and thus poses problems such as supply of heat to the reformer and thermal insulation. Therefore, this fuel cell is not suitable for a portable power source either.
Moreover, a direct methanol fuel cell, in which methanol is used as a fuel and reacts directly at the electrode, is miniaturized easily and expected to be a future portable power source. However, a direct methanol fuel cell causes a reduction in both voltage and energy density due to a crossover phenomenon in which methanol at the negative electrode passes through the solid electrolyte and reaches the positive electrode.
On the other hand, it is also known that hydrogen is generated by a chemical reaction at a low temperature of 100° C. or less and used as a fuel of a fuel cell (Patent Documents 1 to 5). These methods use a metal that reacts with water to produce hydrogen, such as aluminum, magnesium, silicon or zinc, as a hydrogen source.
Patent Document 1: U.S. Pat. No. 6,506,360
Patent Document 2: JP 2566248 B2
Patent Document 3: JP 2004-231466 A
Patent Document 4: JP 2001-31401 A
Patent Document 5: U.S. Pat. No. 6,582,676
Patent Documents 1 to 3 disclose techniques allowing aluminum to react with an alkali or acid. Although these techniques easily can produce hydrogen chemically, the equivalent weight of the alkali or acid corresponding to aluminum needs to be added, which in turn reduces the energy density because a large proportion of the material is other than the hydrogen source. Moreover, the reaction product (oxide or hydroxide) forms a film on the surface of the aluminum, so that water cannot come into contact with the aluminum inside the film. This may lead to a problem that the oxidation reaction stops while only at the surface of the aluminum.
Patent Document 4 is intended to avoid the above problem by removing the film mechanically from the metal surface. However, the device should have mechanical equipment for removal of the film and becomes larger.
In Patent Document 5, alumina is added as a catalyst to suppress the formation of a hydroxide film, and hydrogen is generated at low temperatures. However, it is not possible to generate hydrogen by using only a metal such as aluminum. Moreover, the addition of the catalyst can reduce the content of the metal (aluminum) that serves as a hydrogen source, thus reducing the amount of hydrogen generated.